A Phase I/II Trial of Recombinant-Methionyl Human Stem Cell Factor (SCF) in Adult Patients With Sickling Disorders

Sickle cell anemia is a genetic disorder that results from a single nucleotide substitution in codon 6 of the beta-globin gene which, in the homozygous state, produces an abnormal hemoglobin that is prone to polymer formation when deoxygenated. The polymerized hemoglobin leads to impaired deformability and sickling of red blood cells which subsequently lodge in end-arterioles producing the classic and most prominent feature of the disorder, repeated vasoocclusive crises. Despite knowledge of the precise genetic defect for decades, only recently has there been therapeutic impact based upon this knowledge when a clear benefit from treatment with hydroxyurea, a cell cycle-specific agent administered to induce production of fetal hemoglobin (HbF) by stimulating gamma-globin synthesis, was reported in patients with sickle cell disease (SCD). The reduction in the frequency and severity of vasoocclusive crises seen has been attributed to the increase in HbF levels in responsive patients. While the majority of patients demonstrate a rise in HbF, not all such patients benefit from treatment. Given these results, alternative agents that also stimulate the production of HbF warrant investigation in the treatment of SCD. Recombinant-methionyl human stem cell factor (SCF) is a hematopoietic growth factor with activity on immature hematopoietic progenitor cells. SCF stimulates the production of HbF in vitro and in vivo, and this effect is attainable without the myelosuppression associated with hydroxyurea. In this phase I/II trial, we will administer SCF in a dose escalating fashion to patients with sickling disorders. Parameters to be measured are HbF levels, F cell levels, peripheral blood CD34 levels, frequency, duration, and severity of vasoocclusive crises, and toxicity.